1. Field of the Invention
The present invention relates to a variable speed motor, and more particularly to a variable speed motor for allowing a relay to perform a switching operation between serial/parallel connections of at least one winding composed of a main winding and an auxiliary winding, connecting a tap winding to a 4-pole main winding to perform a switching of the tap winding, such that it can greatly increase a variable range of a rotation speed of the motor.
2. Description of the Related Art
FIG. 1 is an exploded perspective view of a rotor and a stator contained in a conventional outer-rotation motor. FIG. 2 is a circuit diagram of a conventional capacitor-type single-phase induction motor.
Typically, a motor includes a stator to which windings are mounted, and a rotator to which a permanent magnet is mounted. The motor generates periodic current variation in the winding mounted to the stator, torque occurs in the rotor by a constant variation of a magnetic field depending on current variation, such that the motor can acquire rotation power by the torque.
The motor is classified into an inner-rotation motor and an outer-rotation motor according to positions of the stator and the rotor. Particularly, the outer-rotation motor installs the stator 1 into the rotor 2, such that the rotor 2 is rotated by variation in current flowing in the winding of the stator 1, as shown in FIG. 1.
However, a general single-phase induction motor generates an alternating magnetic field caused by a main winding, differently from a three-phase induction motor capable of naturally generating a rotating magnetic field, such that it does not generate rotation force, and a single-phase motor in a stationary state is not rotated alone.
Therefore, the induction motor includes not only a main winding but also an auxiliary winding capable of generating a starting torque, and includes a starting device for initially starting the above-mentioned induction motor. The induction motor is classified into a split-phase start motor, shaded-coil type motor, a capacitor-operation motor, and a repulsion start motor according to categories of the starting device.
For example, the widely used capacitor motor will hereinafter be described with reference to FIG. 2.
FIG. 2 is an equivalent circuit of a conventional capacitor-type single-phase induction motor. Referring to FIG. 2, the capacitor-type single-phase induction motor includes a main winding LM, an auxiliary winding LS, and a capacitor C connected to the auxiliary winding LS in series. If a single-phase AC power signal E is applied to the capacitor-type single-phase induction motor, an alternating magnetic field is generated in the main winding LM. In this case, the capacitor C controls a phase of a current signal flowing in the auxiliary winding LS to be preceded by a predetermined angle of 90°, such that an auxiliary magnetic field having a phase difference of 90° compared with the alternating magnetic field of the main winding LM is generated in the auxiliary winding LS.
Therefore, the alternating magnetic field generated from the main winding LM and the auxiliary magnetic field generated from the auxiliary winding LS have different magnetic field phases, such that they are not compensated, but are summed. As a result, a rotating magnetic field is generated, such that the single-phase induction motor is rotated.
Although the single-phase induction motor is less effective than the three-phase induction motor in terms of performance and cost, its use is easier than that of the three-phase induction motor, so that it is widely used for household appliances requiring mainly low power consumption.
In the meantime, the single-phase induction motor including the starting device may require high-speed rotation or low-speed rotation due to unique operation characteristics of a specific device to which the motor is mounted.
In this case, if the motor is rotated upon receiving a single-phase AC (Alternating Current) voltage signal, a current variation of the received AC voltage signal is constantly maintained, resulting in limitation in changing speed of the motor. As a result, an additional device for varying the speed of the motor is required, such that other devices (for example, a three-phase motor, an inverter circuit, and a starting drive circuit) are also required.
If the motor includes the above-mentioned additional devices, additional production costs associated with the additional devices inevitably occur. In order to solve the above-mentioned problems, there has been newly proposed an improved method for including not only the coil wound on the stator but also an additional tap winding, resulting in variation of the motor speed.
If a user or operator desires to vary the motor speed using the tap winding, there is a limitation in varying the motor speed because a minimum variable speed of the motor corresponds to about a predetermined value of ⅔ of a synchronous speed of the motor, such that it is very difficult to extend the range of the variable speed of the motor.